We have previously described a precisely phased nucleosome core particle generated by in vitro interaction of chicken histones with a 255 bp cloned segment of sea urchin DNA containing a 5 S rRNA gene. We have created a series of mutants of this DNA to examine phasing mechanisms; mutations on the right hand side of the sequence do not affect phasing while those on the left suggest that two distinct regions are involved in histone-DNA interactions. DNA constructions derived from tendem repeats of this sequence and its derivatives are under study as models for higher order chromatin structure. The tandem repeats associate with histone octamers in a manner equally precise to the monomer DNA. Repeats over 12 in length appear to adapt a more compact conformation than those less than that length. Studies of the TRP1/ARS1 yeast episomal chromatin continue. The plasmid has four distinct types of nucleoprotein interactions. Chimaeric plasmids based on this episome are allowing determination of the features involved in both nucleosome phasing and generation of nuclease sensitive sites. We have isolated the plasmid as chromatin in nearly pure form and are now able to investigate both the structural properties of unique yeast genes and the protein components involved in generation of those structures. Studies of the developmentally regulated early histone gene set of sea urchins are completed. Three histone genes are preceeded by nuclease hypersensitive regions; each region consists of a common set of sites, equidistantly spaced and spanning 150 bp of DNA. A number of investigations suggest that these sites are created by proteins interacting with enhancer-like DNA sequences and may be involved in control of genetic activity of this early embryonically expressed gene family.